: The effect of weightbearing exercise on bone mass, bone turnover, and calcium absorption will be assessed over 2 years in women aged 18-30 years. A feature of the proposed study which distinguishes it from others is that the women will be premenopausal. This population, as opposed to a population of postmenopausal women in whom bone loss is already established was selected because it is proposed that an increase in activity in young women is more effective in reducing the risk of osteoporosis. Subjects will be randomly assigned to the exercise program which consists of aerobic, weightbearing exercise. Fitness will be measured by maximal and submaximal oxygen uptake on a treadmill and body composition by underwater weighing and bioelectrical impedance analysis. Bone mass will be evaluated by single and dual photon absorptiometry of the radius, hip, and spine. Bone turnover will be assessed by plasma bone-Gla-protein, bone resorption by fasting urinary excretion of hydroxyproline, and bone formation by serum alkaline phosphatase. If the exercise program does increase bone mass or turnover, it is likely to increase calcium absorption. Calcium absorption will be determined using a stable (nonradioactive) isotope of calcium. A positive effect of exercise on bone mass in young women may have important implications in the prevention of osteoporosis later in life.